Submitted to: Journal of Environmental Quality
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/20/2008
Publication Date: 7/1/2008
Citation: Sharpley, A.N., Kleinman, P.J., Heathwaite, A.L., Gburek, W., Weld, J.L., Folmar, G.J. 2008. Integrating contributing areas and indexing phosphorus loss from agricultural watersheds. Journal of Environmental Quality. 37:1488-1496. Interpretive Summary: Most states in the U.S. have adopted Phosphorus Indexing to guide phosphorus-based management of agricultural fields by identifying the relative risk of phosphorus loss at farm and watershed scales. To a large extent, this risk is based on hydrologic principles whereby frequently occurring storms have a given potential to initiate runoff from fields and transport phosphorus to the stream. In regions where variable source area hydrology dominates, surface runoff generation by “saturation excess” has been linked to distance from stream, with larger events resulting in larger contributing distances. By measuring phosphorus loss from an agricultural watershed in Pennsylvania for 10 years, we found that a few large storms can carry large amounts of phosphorus from the watershed. Modifying the Phosphorus Index to account for these large storms is possible by adjusting the storm-return period threshold, which sets contributing distances in the Index. By increasing the storm-return period threshold, in the Phosphorus Index, it is possible to account for phosphorus contributed from fields more distant from the stream channel. Thus, it is possible to formulate the Index to forecast a predetermined risk of phosphorus loss. However, the increased watershed area impacted relative to a reduced risk of phosphorus loss creates a management policy dilemma that must be faced. In other words, implementation of more conservative or restrictive watershed management strategies for phosphorus, maximize phosphorus loss reductions but may limit certain farm operations even further.
Technical Abstract: Most states in the U.S. have adopted P Indexing to guide P-based management of agricultural fields by identifying the relative risk of P loss at farm and watershed scales. To a large extent, this risk is based on hydrologic principles whereby frequently occurring storms have a given potential to initiate runoff from fields and transport P to the stream. In regions where variable source area hydrology dominates, surface runoff generation by “saturation excess” has been linked to distance from stream, with larger events resulting in larger contributing distances. Thus, storm-return period and P loss from a 39.5 ha mixed-land use watershed in Pennsylvania was evaluated to relate return-period thresholds and distances contributing P to streams. Of 248 storm flows between 1997 and 2006, 93% had a return period of 1 yr, contributing 54% of TP export, while the largest two storms (10-yr return period) accounted for 21% of TP export. Contributing distance thresholds for FD-36 were determined (50 to 150 m) for a range of storm-return periods (1 to 10 yr) from hydrograph analysis. By modifying storm-return period thresholds in the P Index and thereby contributing distance, it is possible to account for greater risk of P loss during large storms. For instance, increasing return period threshold from 1 (current P Indices) to 5 yr, which contribute 67% of TP export, increased the P-management restricted area from 20 to 58% of the watershed. An increase in impacted area relative to a decreased risk of P loss creates a management-policy dilemma, which cannot be ignored.